The present invention relates to downhole recorders for use in oil and gas wells.
Various types of downhole recorders have been developed for use in wells, for example, to measure pressure and temperature. One such well known recorder senses pressure via the expansion and contraction of a Bourdon tube. Data is recorded on a moving chart using a stylus mechanically linked to a moving end of the Bourdon tube. The recorder is large and intricate, having a relatively long length. The large size of the recorder may render it impractical for use in confined spaces and in highly deviated boreholes. When the recorder is retrieved to the surface, it is necessary to convert the analog trace from the chart to a usable format. Typically, the data is converted manually at considerable expense and with substantial delay.
Electronic recorders provide the advantage of small size and are capable of providing the data directly in digital form to data processing instrumentation. However, electronic recorders generally have more limited storage capacity than do mechanical recorders. U.S. Pat. No. 4,033,186 shows a downhole electronic recorder wherein a preprogrammed solid state clock initiates measurement sequences and deactivates the circuitry between sequences. A time delay is programmed into the clock so that the first reading sequence is not initiated until the gauge has been inserted into the well shaft to a desired depth.
In well testing and completion activities, the wall of the borehole is perforated, for example, to test the producing capability of a formation, or to bring a well into production. The wall is perforated typically with the use of a perforating gun which is either suspended in the well on a wireline or is run into the well on tubing. Especially where the gun is tubing conveyed, considerable time and expense are required to run in the guns, and it is desireable to reliably determine that the perforating guns have been successfully actuated.
Where a wireline gun is used to perforate a well, a sensor in the gun can be coupled to surface equipment by wire line in order to detect and convey signals indicative of gun firing. Such techniques utilize, for example: (1) an inertial switch disposed within the perforating gun and arranged to interrupt the electrical gun firing circuit in response to gun recoil from firing; (2) an accelerometer disposed within the perforating gun and arranged to generate an electrical signal in response to recoil motion of the perforating gun; and (3) a downhole microphone (geophone) arranged to convey the sound of the perforating gun to a speaker at the surface.
However, in the use of tubing conveyed perforating guns, there typically is no electrical conductor extending from the surface downhole to the gun, and the above mentioned techniques cannot be utilized for detecting its firing. In one known technique, an explosive device is attached to one end of a perforating gun which is actuated from an opposing end. When the gun is actuated, only the complete detonation of a detonating cord within the gun from the first end thereof to the second, where the explosive device is located, will suffice to actuate the explosive device. The explosive device implements a time delay so that complete detonation of the perforating gun is followed in time by several seconds by the actuation of the explosive device. A sensor at the wellhead detects energy produced by the firing of the perforating gun and, subsequently, energy from the firing of the explosive device, so that it can be reliably determined at the wellhead whether the gun has fired completely. This signalling technique works quite well under most circumstances. However, in environments where a great deal of background noise is present, for example, on a floating rig, the surface noise tends to obscure the signals from the perforating gun and the explosive device.